Pathway toward Scalable Energy-Efficient Li-Mediated Ammonia Synthesis

Lithium-mediated ammonia synthesis (LiMAS) is an emerging electrochemical method for NH₃ production, featuring a meticulous three-step process involving Li⁺ electrodeposition, Li nitridation, and Li₃N protolysis. The essence lies in the electrodeposition of Li⁺, a critical phase demanding current oscillations to fortify the solid-electrolyte interface (SEI) and ensure voltage stability. This distinctive operational cadence orchestrates Li nitridation and Li₃N protolysis, profoundly influencing the NH₃ selectivity. Increasing N₂ pressure enhances the NH₃ faradaic efficiency (FE) up to 20 bar, beyond which proton availability controls selectivity between Li nitridation and Li₃N protolysis. The proton donor, typically alcohols, is a key factor, with 1-butanol observed to yield the highest NH₃ FE. Counterion in the Li salt is also observed to be significant, with larger anions (e.g., exemplified by BF₄^-) improving SEI stability, directly impacting LiMAS efficacy. Notably, we report a peak NH₃ FE of ∼70% and an NH₃ current density of ∼-100 mA/cm² via a delicate balance of process conditions, encompassing N₂ pressure, proton donor, Li salt, and their respective concentrations. In contrast to the recent literature, we find that the theoretical maximum energy efficiency of LiMAS hinges significantly on the proton source, with LiMAS utilizing H₂O calculated to have a maximum achievable energy efficiency of 27.8%. Despite inherent challenges, a technoeconomic analysis suggests high-pressure LiMAS to be more feasible than both ambient LiMAS and a modified green Haber-Bosch process. Our analysis finds that, at a 100 mA/cm² NH₃ current density and a 6 V cell voltage, LiMAS delivers green NH₃ at an all-inclusive cost of $456 per ton, significantly lower than conventional cost barriers. Our economic analysis underscores high-pressure LiMAS as a potentially transformative technology that may revolutionize large-scale NH₃ production, paving the way for a sustainable future.